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ARCHIVED: What are bits, bytes, and other units of measure for digital information?

A bit is a binary digit, the smallest increment of data on a computer. A bit can hold only one of two values: 0 or 1, corresponding to the electrical values of off or on, respectively.

Because bits are so small, you rarely work with information one bit at a time. Bits are usually assembled into a group of eight to form a byte. A byte contains enough information to store a single ASCII character, like "h".

A kilobyte (KB) is 1,024 bytes, not one thousand bytes as might be expected, because computers use binary (base two) math, instead of a decimal (base ten) system.

Computer storage and memory is often measured in megabytes (MB) and gigabytes (GB). A medium-sized novel contains about 1 MB of information. 1 MB is 1,024 kilobytes, or 1,048,576 (1024x1024) bytes, not one million bytes.

Similarly, one 1 GB is 1,024 MB, or 1,073,741,824 (1024x1024x1024) bytes. A terabyte (TB) is 1,024 GB; 1 TB is about the same amount of information as all of the books in a large library, or roughly 1,610 CDs worth of data. A petabyte (PB) is 1,024 TB. 1 PB of data, if written on DVDs, would create roughly 223,100 DVDs, i.e., a stack about 878 feet tall, or a stack of CDs a mile high. Indiana University is now building storage systems capable of holding petabytes of data. An exabyte (EB) is 1,024 PB. A zettabyte (ZB) is 1,024 EB. Finally, a yottabyte (YB) is 1,024 ZB.

Many hard drive manufacturers use a decimal number system to define amounts of storage space. As a result, 1 MB is defined as one million bytes, 1 GB is defined as one billion bytes, and so on. Since your computer uses a binary system as mentioned above, you may notice a discrepancy between your hard drive's published capacity and the capacity acknowledged by your computer. For example, a hard drive that is said to contain 10 GB of storage space using a decimal system is actually capable of storing 10,000,000,000 bytes. However, in a binary system, 10 GB is 10,737,418,240 bytes. As a result, instead of acknowledging 10 GB, your computer will acknowledge 9.31 GB. This is not a malfunction but a matter of different definitions.

We count in base 10 by powers of 10:

101 = 10 102 = 10*10 = 100 103 = 10*10*10 = 1,000 106 = 1,000,000

Computers count by base 2:

21 = 2 22 = 2*2 = 4 23 = 2*2*2 = 8 210 = 1,024 220 = 1,048,576

So in computer jargon, the following units are used:

Unit Equivalent
1 kilobyte (KB) 1,024 bytes
1 megabyte (MB) 1,048,576 bytes
1 gigabyte (GB) 1,073,741,824 bytes
1 terabyte (TB) 1,099,511,627,776 bytes
1 petabyte (PB) 1,125,899,906,842,624 bytes

Note: The names and abbreviations for numbers of bytes are easily confused with the notations for bits. The abbreviations for numbers of bits use a lower-case "b" instead of an upper-case "B". Since one byte is made up of eight bits, this difference can be significant. For example, if a broadband Internet connection is advertised with a download speed of 3.0 Mbps, its speed is 3.0 megabits per second, or 0.375 megabytes per second (which would be abbreviated as 0.375 MBps). Bits and bit rates (bits over time, as in bits per second [bps]) are most commonly used to describe connection speeds, so pay particular attention when comparing Internet connection providers and services.

This document was developed with support from National Science Foundation (NSF) grant OCI-1053575. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF.

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Last modified on January 07, 2013.

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